In an x-ray diffraction system the function of x-ray optics is to condition the primary x-ray beam into the required wavelength, beam focus size, beam profile and divergence. One type of x-ray optics device is the total external reflection mirror. Total external reflection happens when x-rays strike on a polished surface at a small grazing incident angle. The reflected x-rays from the surface take off at the same angle as the incident angle. The polished surface behaves similarly to a mirror reflecting visible light. Therefore such a mirror is referred as an x-ray mirror. The reflecting mirror is made of materials with refractive index less than unity. The total external reflection can only be observed at an incident angle less than the critical incident angle θC. The value of the critical angle is dependent on the wavelength of the x-ray radiation and the reflecting materials. For a typical laboratory x-ray source, the wavelength is in the range of a fraction of nanometer, and the critical angle is in the range of a fraction of a degree to several degrees.
Another type of x-ray optics device is the multilayer mirror. A multilayer mirror consists of alternating layers of heavy materials as reflection layers and light materials as spacer layers. A multilayer mirror works on the same principle as Bragg diffraction from a natural crystal, selectively reflecting certain wavelengths based on the spacing between the mirror layers. In this way, multilayer mirrors can be used as monochromators. In contrast to a natural crystal, a multilayer mirror typically has larger d-spacing so that the incident angle and the diffracted angle are typically only a few degrees. In these mirrors, the number of layers, the d-spacing of the layers, and the distribution of the layer thickness can be varied to modify the mirror performance.